As a rule, a fastener should be no less durable than the material it is fastening. With secret fixed fasteners over a benign internal environment, this is easily achieved.
The potential for air-borne salt ingress into the cavity of a tray roof is low; the pan is flat or nearly flat, the ribs are narrow and closed at the ends. The BRANZ publication, Research Now: Roof Ventilation 2, tested the durability of metals inside a vented and unvented ceiling space. Even in this Extreme Marine environment, the corrosion rate was low. Their conclusion was that the installation of ventilation openings to roof cavities should not have a negative effect on the long-term performance of galvanised metal fixtures in these spaces. In the unvented spaces, the corrosion rate was even lower.
Compatibility of materials should also be considered. Aluminium, zinc, and aluminium/zinc alloys are compatible in contact. While not normally considered compatible, stainless steel clips have also been proven to be durable, and not affect the durability performance of aluminium, zinc, aluminium/zinc alloys, or copper, in a dry internal environment.
Over wet and corrosive internal environments, or where the underside of a roof or canopy is exposed to weather from underneath, both the durability of the clip and compatibility of the roof and cladding material must be assessed in accordance with the micro-environment.
In all cases, adequate ventilation of the roof or wall is essential to prevent degradation of the cladding and other building elements.